Conventionally, the reference voltage generation circuit which generates a constant voltage irrespective of a variation in supply voltage or temperature has been widely used in analog circuits, such as analog-digital converters, and the like.
An example of such a reference voltage generation circuit, illustrated in FIG. 7, has a band gap reference circuit 30 (hereinafter, abbreviated as “BGR circuit”). The BGR circuit 30 includes two diode elements D1 and D2 which have different current densities, three resistive elements R1, R2 and R3, a P-type first transistor Tr1 for supplying a current to a reference voltage output terminal O, a P-type second transistor Tr2 which determines a drain current flowing through the first transistor Tr1 by a current mirror structure, and a feedback type control circuit 31. The feedback type control circuit 31 is formed by a differential amplifier circuit 32 and an N-type transistor Tr3 and controls the drain current of the second transistor Tr2.
The feedback type control circuit 31 functions to stabilize the operation at a stabilization point at which the voltage at a node N1 (the connection point between the anode of the diode D1 and the resistive element R3) is equal to the voltage at a node N2 (the connection point between the resistive elements R1 and R2). For example, if the voltage at the node N1 is higher than that of the node N2, the gate voltage of the transistor Tr3 is increased by the differential amplifier circuit 32, so that the drain current of the transistor Tr3 is increased, and the drain current of the second transistor Tr2 is increased. As a result, the drain current I1 of the first transistor Tr1 is increased so that the output voltage of the reference voltage output terminal O is increased, and accordingly, the operation shifts to the operation stabilization point. On the contrary, if the voltage at the node N1 is lower than that of the node N2, the gate voltage of the transistor Tr3 is decreased by the differential amplifier circuit 32, so that the drain current of the transistor Tr3 is decreased, and the drain current of the second transistor Tr2 is decreased. As a result, the drain current I1 of the first transistor Tr1 is decreased so that the output voltage of the reference voltage output terminal O is decreased, and accordingly, the operation shifts to the operation stabilization point.
FIG. 8 illustrates the relationship between the output voltage of the reference voltage output terminal O and the voltages at the nodes N1 and N2. As seen from FIG. 8, the voltage at the node N1 is substantially constant so long as the voltage of the reference voltage output terminal O is equal to or higher than a predetermined output voltage irrespective of the output voltage value at the reference voltage output terminal O. Meanwhile, the voltage at the node N2 increases as the output voltage of the reference voltage output terminal O increases. Therefore, the plots of the voltages at the nodes N1 and N2 have an intersection point (normal stabilization point). The feedback type control circuit 31 allows the BGR circuit 30 to operate at the normal stabilization point. As a result, the BGR circuit 30 is enabled to generate an output voltage independent of the supply voltage.
The output voltage of the reference voltage output terminal O during the operation at the normal stabilization point is expressed by the following formula:Output Voltage=Vd+kT/q·R2/R1·log(Is2/Is1·R2/R3)where Vd is the voltage across the diode D1, k is Boltzmann constant, T is temperature, q is the amount of charges of electrons, and Is1 and Is2 are the saturation currents of the diode elements D1 and D2, respectively. Since voltage Vd across the diode D1 has a negative temperature characteristic, an output voltage independent of the temperature can be generated by setting Voltage Vd such that the resistance values of the resistive elements R1, R2 and R3 or the temperature characteristics of currents Is1 and Is2 of the diode elements are canceled with respect to the temperature characteristic of voltage Vd across the diode D1.
As described above, the BGR circuit 30 possesses the advantage of generating an output voltage independent of the supply voltage and temperature. However, as can be seen from FIG. 8, if the output voltage of the reference voltage output terminal O is in the lower range, drain current I1 supplied from the first transistor Tr1 is very small, and there is an abnormal stabilization point at which the plots of the voltages at the nodes N1 and N2 are crossing. Thus, the feedback type control circuit 31 sometimes controls the operation of the BGR circuit 30 at this abnormal stabilization point. In such a case, the output voltage of the reference voltage output terminal O is near the ground potential so that a desired voltage is not output. In view of such, the reference voltage generation circuit shown in FIG. 7 includes a start-up circuit 40 for the BGR circuit 30. When the operation is stabilized at the abnormal stabilization point, the start-up circuit 40 allows the BGR circuit 30 to shift its operation from the abnormal stabilization point to the normal stabilization point.
The conventional start-up circuit 40 has a structure as described in Patent Document 1. A current is supplied from a constant current source 15 to a diode element 16 to generate a diode voltage which is used for determining the normal stabilization point. This diode voltage is compared with the output voltage of the reference voltage output terminal O by a comparator circuit 17. If the output voltage of the reference voltage output terminal O is lower than the diode voltage, it is determined that the operation is at the abnormal stabilization point. Accordingly, a P-type transistor 18 is turned ON, and the drain current of the transistor 18 is supplied to the reference voltage output terminal O, whereby the output voltage is increased. Alternatively, a start-up circuit described in Patent Document 2 includes a voltage monitoring circuit for monitoring an output voltage. In this structure, if the voltage monitoring circuit determines that the operation is at the abnormal stabilization point, voltages are supplied to the reference voltage output terminal O and the node N1.
Thus, in the conventional start-up circuits, the output voltage of the reference voltage output terminal O is monitored all the time. In case of abnormal conditions, the voltage of the reference voltage output terminal O is increased such that the operation is shifted from the abnormal stabilization point to the normal stabilization point.                Patent Document 1: Gazette of Japanese Patent No. 3422706 (FIG. 1)        Patent Document 2: Gazette of Japanese Patent No. 3185698 (FIG. 1)        